1. Field of Invention
The present invention relates to hot-melt sealing glass compositions and methods of making and using the same.
2. Description of Related Art
FIG. 1 shows a schematic cross-section view of an exemplary microelectromechanical systems (“MEMS”) device 10 formed in or on a device wafer 20 made of silicon or glass. The MEMS device 10 could be an accelerometer, rate sensor, actuator, pressure sensor etc. Signal lines 30, a portion of which may be formed in the device wafer 20, electrically connect the MEMS device 10 to a microprocessor and/or to other circuitry (not shown). A cap wafer 40 made of silicon or glass is bonded to the device wafer 20 using a sealing glass composition, which is melted and re-solidified to form a hermetic glass seal 50 between the cap wafer 40 and the device wafer 20. The cap wafer 40, the hermetic glass seal 50 and the device wafer 20 thus cooperate to define a package comprising a cavity 60 within which the MEMS device 10 is enclosed and protected.
By their very nature of operation, MEMS devices must be free to move to some degree. Thus, the seal between the device wafer and the cap wafer must prevent dust, moisture and other foreign matter that could interfere with the function of the MEMS device from entering the cavity. Some MEMS devices, such as pressure sensors, for example, require that the cavity be completely evacuated and hermetically sealed. Some MEMS devices, such as motion sensors and accelerometers with resonating micromachined components for example, operate more effectively in a vacuum. And some MEMS devices need a gas back-filled to create a certain atmosphere. A hermetical seal between the cap wafer and the device wafer also ensures that moisture is excluded from the cavity, which could lead to the formation of ice crystals at low temperatures and/or otherwise impede the operation of the MEMS device.
Sealing glass compositions used in MEMS device fabrication are typically applied using screen printing techniques, in which the sealing glass composition is deposited in the form of a paste that contains a particulate glass frit material, a thixotropic binder, and a solvent for the binder. The proportions of glass frit, binder and solvent are adjusted to allow screen printing of a controlled volume of the paste on a designated bonding surface of one of the wafers, typically on the cap wafer. After drying, burning out the binder (BBO) and pre-glazing, which removes all of the organic components from the glass frit bonding paste, the cap and device wafers are aligned and then mated so that the glass frit particles contact complimentary bonding surfaces. The wafers are then incrementally heated to remelt, flow and impart wetting of the wafer surfaces by the glass frit so that upon cooling, the glass frit material re-solidifies to form a substantially homogeneous glass bond line between the wafers.
Because the glass frit particles in glass frit bonding pastes are dispersed in a binder and solvent system, the glass frit particles have a tendency to flow out (i.e., spread) somewhat after they are screen printed and until the solvent is removed. For example, a screen printed line of glass frit bonding paste having an initial line width of about 160 microns will typically spread out to a width of about 200 microns before the solvent is removed from the paste. It would be advantageous if the initial line width could be maintained and if the drying step could be eliminated.